DOC: strings longer than min_itemsize are not truncated

Fixed docs on min_itemsize

Author: Pietro Battiston <[email protected]>

Closes #12251 from toobaz/minitemsizedoc and squashes the following commits:

4009789 [Pietro Battiston] DOC: strings longer than min_itemsize are not truncated

Author: toobaz

doc/source/io.rst

@@ -2902,8 +2902,8 @@ Storing Mixed Types in a Table
 ++++++++++++++++++++++++++++++
 
 Storing mixed-dtype data is supported. Strings are stored as a
-fixed-width using the maximum size of the appended column. Subsequent
-appends will truncate strings at this length.
+fixed-width using the maximum size of the appended column. Subsequent attempts
+at appending longer strings will raise a ``ValueError``.
 
 Passing ``min_itemsize={`values`: size}`` as a parameter to append
 will set a larger minimum for the string columns. Storing ``floats,